As far as the board is concerned its merely preference... do you need SLI/crossfire board that will bridge 3 cards together or is 1 powerhouse graphics card enough? Problem with bridging with SLI is you need the same exact card in order for it to work or else its non-sli. For example if you buy a $500 Nvidia GTX 280 then you need to buy another $500 Nvidia GTX 280 to run it in SLI.
As far as raw performance is concerned here's a list of some chips you might want to consider:
Intel Core i7 980x $999.99 Passmark Score 10,189
Features: Six physical cores with HT (12 logical CPU cores), QPI link 6.4GT/s, 12MB L2 cache
AMD Phenom II X6 1090T Black Edition $295.99 6,499
Features: Six physical cores, 4000MHZ hypertransport bus, 3MB L2 cache, 6MB L3 cache
Intel Core i7-875K $329.99 Passmark Score 6,509
Features: Quad core processor with HT (8 logical CPU cores), 8MB L3 cache
Or you could hit the lottery and buy a 1567 socket server board and throw two Intel Xeon x7560s with 24MB cache and 16 physical cores, (32 logical), 16GB of RAM or so... throw in a nice graphics card, some caviar (not the eating kind) ... or a 15,000k RPM raptors in RAID 6... meh it'll only set you back about $15,000. WHAT THE HECK!